The present invention refers to a method of combustion of a fuel in a combustion chamber enclosing a fluidized bed, comprising the steps of: supplying an oxygen-containing gas to the bed from beneath; supplying a fuel and an absorbent to the bed; collecting combustion gases formed during the combustion; purifying the combustion gases by separating solid material therefrom; and recirculating the solid material separated to the combustion chamber through a channel. Furthermore, the invention refers to a combustion plant comprising: a combustion chamber, which is provided to enclose a fluidized bed and in which a combustion of a fuel is intended to be performed while forming combustion gases and during the supply of an absorbent; and a purification device for purifying said combustion gases, said purification device comprising a separating member, arranged to separate particulate material from said combustion gases, and a channel, connecting the separating member and the combustion chamber and being arranged to recirculate the material separated to the combustion chamber.
It is known to combust different fuels in a bed of particulate, incombustible material, which bed is supplied with combustion air from beneath through nozzles in such a manner that the bed becomes fluidized. There is a difference between different types of such combustion in a fluidized bed, which operate according to different principles and under different conditions. Firstly, there is a difference between an atmospheric bed and a pressurized bed. In comparison with an atmospheric bed, a pressurized fluidized bed is characterized by a small plant size in relation to the effect produced, by a high efficiency, and in that the combustion occurs under advantageous conditions from an environmental and economical point of view. A pressurized bed may have a larger height than atmospheric bed since one may operate with greater pressure drops. Among the atmospheric beds, so-called circulating beds are frequently used, in which the bed material is permitted to circulate through a separating device in order to be recirculated to the bed. This enables unburnt fuel to be recirculated, which improves the efficiency of the combustion, as well as the absorbent material not used for absorption of contaminants, sulfur, which decreases the discharge from the combustion. However, such circulating beds operate with relatively high fluidizing velocities, in typical cases in the order of 5-12 m/s. Fluidizing velocity is the velocity that the gas would have had if it would have flowed through the combustion chamber without the pressure of particles. This high velocity causes problems with erosion of for instance the steam tube arrangement provided in bed in such a way that the lifetime thereof significantly decreases. Furthermore, one may discern the so-called bubbling beds in which the fluidizing velocity is relatively low, in typical cases between 0.5 and 2 m/s. Such a bed is relatively well defined in a vertical direction and there is formed a space, called a freeboard, in the combustion chamber above the bed. In this freeboard a relatively small amount of dust particles are present in comparison with a circulating bed but there is essentially no pressure drop across the freeboard.
In recent time some have tried to provide a certain circulation in pressurized beds by supplying the combustion cases leaving the combustion chamber to a cyclone for separation of solid material, which is recirculated to the combustion chamber. In order to obtain completely the desired effect concerning the degree of utilization of the absorbent and the combustion efficiency by the recirculation, the solid material should be supplied at the bottom of the fluidized bed. This means that one has to overcome the pressure drop which is present in the bed and in the cyclone, in typical cases about 0.5 bars.
In order to overcome this pressure drop it has been suggested to provide a dosing device, for example of a cell feeding type, at the end of a recirculating pipe provided preferably vertically and connecting the cyclone to a combustion chamber. The dosing device may comprise a rotatable shutter provided on the pipe and having a weight which in normal cases keeps the shutter in a closed position. When the amount of material in the pipe is sufficient the weight thereof will overcome the weight of the shutter which means that the shutter is opened and the material is discharged. Such a device leads to an intermittent recirculation of solid material. However, such devices do not function in the way intended in the environment of a fluidized bed due to the movements occurring in the bed and the forces caused by these movements. Furthermore, such devices are rapidly destroyed due to the aggressive, erosive and corrosive environment.
Another solution is an L-valve located in the bed and having a vertical portion in which a column of material is built up. In order to provide a flow of material through the channel such a device requires that gas is injected in the lower portion of the L-valve and in order to provide stability it is necessary to continuously measure the height of the column of material, which is very difficult, if not impossible, in the actual environment.
SE-B-460 148 suggests another way of overcoming this pressure drop. SE-B-460 148 discloses a combustion plant having a combustion chamber enclosing a pressurized fluidized bed for the combustion of a fuel while forming combustion gases. Furthermore, the plant comprises a purification of said combustion gases in several stages. In the stage particulate material is separated by means of a cyclone from the combustion gases and supplied to a collection chamber beneath the cyclone. Via a horizontal recirculating channel, the collected dust particles are fed back to the combustion chamber in order to improve the use of unburnt fuel and absorbent material. The recirculation is accomplished by means of an air driven ejector blowing the material into the combustion chamber. However, such an air injection is very expensive. The increase in the absorbent utilization and the combustion efficiency is lost for the compressor providing primary air to the ejector. In addition, this method leads to erosion.
It should be noted that the recirculation of solid material separated from the combustion gases means that the recirculated fine part may provide as much as 10-40% of the mass of the bed, which strongly influences the heat transfer coefficient to the tubes located in the bed. The fine part is comprised of particles having a largest diameter of about 300-400 xcexcm and an average particle diameter of about 50-150 xcexcm.
U.S. Pat. No. 4,021,184 discloses a combustion plant developed for the combustion of waste material. The plant comprises a combustion chamber for a recirculating fluidized bed. The bed disclosed in this document is not pressurized but the plant operates at atmospheric pressure and is of a diluted type (dilute phase fluidized bed), i.e. the fluidized bed fills up the whole combustion chamber. Such a type of bed means that a very large part of the solid, hot bed material will be transported out from the combustion chamber together with the combustion gases formed during the combustion.
Therefore, it is suggested that cyclones for separating dust particles from these gases are provided at the outlet of the combustion chamber and that the separated, hot dust particles are recirculated to the combustion chamber via conduit pipes connecting the cyclones with the combustion chamber. In such a manner it is possible to recover the heat energy in the dust particles leaving the combustion chamber. Thus, a recirculation may be obtained due to the low pressure drop across the bed, i.e. the whole combustion chamber. In addition, the valve mentioned (trickle valve) in the end of the conduit pipe is probably necessary.
EP-B-176 293 discloses another combustion plant having a combustion chamber which encloses a fluidized bed and in which combustion of a fuel is intended to be performed while forming combustion gases. The bed is of a bubbling type but the combustion chamber operates at atmospheric pressure. Furthermore, the plant comprises a cyclone for separating particulate material from the combustion gases and provided above the combustion chamber. The particulate material separated is conducted via a pipe back into the bed by letting the material simply fall freely through the pipe. This is possible since the bed disclosed in this document has a relatively low height, about 1 m. Thereby, also the pressure drop is relatively small.
U.S. Pat. No. 4,103,646 discloses a plant comprising two combustion chambers, the first combustion chamber having a fast fluidized bed, i.e., the fluidizing velocity is between 7 and 10 m/s, and the second combustion chamber having a xe2x80x9cslowxe2x80x9d, bubbling fluidized bed. The combustion gases formed in the first combustion chamber are conducted to a cyclone, where solid material is separated and fed to the second combustion chamber. In the bottom of the second combustion chamber there is a discharge channel for solid material which by means of air injection then is recirculated to the first fast combustion chamber.
By recirculating the separated material to the bed a higher degree of utilization of the absorbent supplied to the combustion chamber is obtained since its time of presence in the process may be prolonged.
The object of the present invention is to provide a method and a combustion plant by which the discharge of undesirable substances may be reduced. In particular, the invention aims at a higher degree of utilization of the absorbent supplied to the combustion for absorbing undesirable substances.
This object is obtained by the method initially defined and characterized in that a gaseous medium in a controlled manner is supplied to the separated solid material present in the channel in order to displace the combustion gases and provide a chemical reaction. During heating of the absorbent, which for instance may be present in the form of a natural lime-containing substance such as limestone or dolomite, in the combustion chamber carbon dioxide evaporates from CaCO3, thereby forming quick lime. This has very good ability of binding sulphur, released during the combustion, thereby forming gypsum. However, in order to be able to obtain large quantities of quick lime, the partial pressure of carbon dioxide has to be low in the gas surrounding the lime-containing absorbent. By the supply in a controlled manner, in accordance with the present invention of a gaseous medium to the separated solid material present in the channel, the partial pressure of the carbon dioxide may be lowered in this channel in such a manner that the absorbent present in the separated material reacts and forms quick lime. Thus, by means of the present invention, the ability of the absorbent to absorb undesirable substances such as sulphur is improved to a high extent. Consequently, by the method according to the invention a reduction of the discharges of undesirable substances may be obtained. The method according to the invention also is suitable for absorption of for instance alkali compounds, chlorine compounds, chlorine and heavy metals.
According to an embodiment of the invention, the gaseous medium is preheated prior to being supplied to the separated material in the channel. In such a manner, the environmental capacity of the plant may be further improved since the ability of the absorbent to evaporate carbon dioxide and absorb sulphur or other undesirable substances increases as the temperature increases. Preferably, the gaseous medium is preheated to a temperature of about 600-900xc2x0 C., preferably 700-870xc2x0 C.
According to a further embodiment of the invention, the gaseous medium comprises air. This is a very advantageous embodiment of the invention, which permits a simple structure of the gas supply according to the invention. The gaseous medium may also comprise steam. Water forms calcium hydroxide together with quick lime. Furthermore, the gaseous medium may comprise nitrogen or a combination of at least two of said substances.
According to a further embodiment of the invention, the gaseous medium is supplied to the channel at a plurality of levels of different height. Thereby the degree of utilization of the absorbent may be further improved. In such a preferred application, air and/or nitrogen is supplied to the channel at a higher level and steam is supplied to the channel at a lower level.
According to a further embodiment of the invention, the absorbent comprises a lime-containing material, for example limestone or dolomite.
According to a further embodiment of the invention, the separated material is supplied to a channel in such a manner that a column of material is formed therein and that the column of material merely due to its weight recirculates the separated material in a continues flow through a passage having a constant opening area and being provided in the lower portion of the channel. Thus, the discharge of the material into the combustion chamber is performed merely by the weight of the column of material and without any influence from outside by previously used auxiliary means such as ejectors or the like. The operation of the plant ensures that the column of material is filled from above through the separating member. Advantageously, the height of the column of material so formed exceeds the height of the bed. Furthermore, the gas from beneath is prevented from entering the channel. In such a manner no fluidization of the material present in the channel may take place and the recirculation of the material will not be hindered.
The object mentioned above is also obtained by the combustion plant initially defined and characterized by at least one member connected to the channel and arranged to supply a gaseous medium to the separated material present in the channel.
There are several preferred embodiments of the combustion plant of the present invention.
Advantageously, heating members are arranged to preheat the gaseous medium. The heating member may comprise a heat exchanger provided in the combustion chamber.
According to a preferred embodiment, the channel comprises an enlarged portion arranged to provide an increased volume to the column of material. By such an enlarged portion, the time of presence of the separated material in the column of material may be further prolonged.
According to a further preferred embodiment of the invention, the gas supply member comprises at least one gas feeding device provided in wall of the channel. Advantageously, this device may comprise a cylinder provided around the channel and having an upper and lower limiting wall so that a closed annular space is formed between the channel and the cylinder and the wall of the channel may comprise a passage between the interior of the channel and the annular space.
According to a further advantageous embodiment, the channel comprises passive means which are provided in such a manner that a column of material is formed in the channel during the operation of the combustion plant and which are forming, in the lower part of the channel, a passage which permits that the weight of the column of material discharges the material therethrough in a continues flow. The flow area of the passage may be constant. Thus, merely the weight of the column of material will provide a continuous and uniform recirculation of separated solid particulate material into the combustion chamber. Since the recirculating device according to the invention comprises passive means not requiring any compressor or other drive means for overcoming the pressure difference and feed out the material from the column of material, the cost of the device is favourable with respect to manufacture as well as operation. Furthermore, the problems of erosion in connection with ejector feeding of the material are avoided. Thus, since the recirculating channel does not comprise any movable structural elements, the reliability thereof is very high.
Furthermore, the passive means may comprise a surface which is provided at the lower end of the channel and seen from beneath covers at least a larger part of the cross-section area of the channel. This surface may form an angle of inclination to a vertical axis which amounts to about 20-90xc2x0, preferably 21-39xc2x0. By such a sloping surface, gas is prevented from entering the channel and at the same time the surface facilitates the recirculation of the material into the combustion chamber. The surface having such a favourable angle of inclination will function as sliding surface or a type of chute for the material.